The present invention relates generally to power conversion systems and their method of operation, and more particularly to an improved firing control circuit for generating predetermined sequences of the firing pulses for initiating conduction of the controlled rectifiers (thyristors) utilized in a bridge converter system.
Power conversion systems and their method of operation for supplying a d.c. load from an a.c. source is well known. It are also known that these systems are adapted to control the magnitude of the d.c. output voltage and have the ability to operate in more than one operational quadrant, a six pulse bridge converter being a typical example. Such a circuit, however, is characterized by a relatively high ripple content in the load voltage and further requires that full control be maintained of all the rectifiers of the bridge arrangement throughout the operating range. The power factor of this arrangement is also known to be relatively poor at lower output voltages.
When the load is inductive and the load current tends to be continuous as in the case for a motor load, the addition of a free wheeling diode configuration is known to reduce load voltage ripple and to improve the power factor at lower output voltages. Such converters as these, however, do not permit fourth quadrant operation and the degree of control remains limited. These principles as well as others relating to known a.c. to d.c. power conversion systems are set forth in detail in "Thyristor Phase-Controlled Converters and Cyclo-Converters," by B. R. Pelly, Copyright 1971, by John Wiley and Sons, Inc. (Library of Congress Catalogue No. 70-127276). Reference to this work can be made when desirable for a more exhaustive treatment of the subject.
It is also known that the input power factor of a six pulse phase control thyristor bridge converter supplying an inductive load can be improved at low voltage by adding two additional neutral leg thyristors which are connected from the positive and negative buss connected across the load to the neutral point of the a.c. system. Such a configuration as well as the basic circuit concepts therefor are set forth in a the new abandoned patent application, U.S. Ser. No. 748,281, entitled "Power Conversion System and Method of Control," filed on Dec. 7, 1976, by the assignee of the present invention in the name of David L. Lafuze, which application is specifically incorporated herein by reference.
As will be shown hereafter, when reference to the drawings in this application is made, when a bridge converter such as disclosed in the abandoned Lafuze application is operating at a low d.c. output voltage and with continuous current in the first quadrant, there are intervals in the cycle when the phase leg thyristors are conductive for less than 60.degree. in a three phase system and that the respective thyristors in both neutral legs are simultaneously conductive. Flat spots indicative of zero output voltage appear on the d.c. load voltage waveform, since the a.c. power system is being by-passed, and as a result, an undesirable transport lag or time delay will exist during voltage reversal as the system makes a transition from first to fourth quadrant operation.